To determine the force acting on a force-measuring device, the latter is normally equipped with a force receiver, a force-transmitting portion, and a measurement transducer. In this arrangement, the input force is received by means of a force receiver and passed on by way of the force-transmitting portion to the measurement transducer. For example in a weighing device, the input force is represented by the weight force of the weighing object. The force receiver serves to accept the input force and is typically realized in the form of a measuring plate, a weighing pan, or a weighing platform. The force-transmitting portion conveys the force from the force receiver to the measurement transducer and is configured for example as a rod, a lever mechanism, or a load-supporting device.
The path taken by the force from the force receiver through the force-transmitting portion to the measurement transducer defines a force flow path. In the example of a balance, the force flow path follows the direction of gravity, i.e., it proceeds vertically from top to bottom.
The measurement transducer is a mechanical-to-electrical converter that transforms the input quantity, i.e., a force, into a corresponding electrical measurement signal. Accordingly, in a weighing device the measurement transducer produces an electrical measurement signal that corresponds to the weight force exerted by the weighing object. This electrical measurement signal is normally passed on through a signal-processing unit to an indicator unit or to a further processing device, for example a system controller.
In order to obtain the highest possible measurement accuracy, it is important that, as far as possible, none of the force conveyed through the force-transmitting portion is lost in the transmission. Losses can occur as a result of a mechanical short circuit, a so-called force shunt, if portions of the force that is to be transmitted are drained off through parasitic mechanical connections. These portions will consequently not be received by the force transducer, so that considerable measurement errors can occur as a result.
A force shunt occurs if movable parts of the force-transmitting portion come into contact with other objects or with persons in such a way that the free mobility of the force-transmitting portion in the direction of the force flow path is restricted. This is the case for example if during the weighing process the movable transmitting rod comes into contact with the stationary rim of a passage opening for the transmitting rod.
The terms “mobility” and “movement” of the force-transmitting portion are distinguished from each other in that the former relates to a mechanical relationship between adjacent parts, while the latter relates to a change in spatial position. For example, in a weighing device disclosed in EP 0 254 594 the mobility of the force transmission is achieved through flexure-pivoted beams. A sensor, which is arranged on one of the beams, serves to detect extraneous disturbances as abnormal accelerations. However, what is being monitored is not the mobility of the beam as enabled for example by the flexibility of its pivotal connections. Rather, the latter is considered as a given, and according to the teachings of EP 0 254 594, lower detected values of acceleration are interpreted as a lower level of disturbances.
Furthermore, accumulations of dust on the force-transmitting portion and on the stationary parts can lead to so-called dust bridges. These dust bridges can build up over time and, without being noticed, can lead to force shunts and thus to measurement errors.
The known state of the art offers a variety of methods to avoid a force shunt. For example, in a balance disclosed in DE 102 53 601, measurement errors due to dust accumulations in the area between the stationary housing and the vertically movable force-transmitting member can be prevented by a stream of gas directed away from the movable force-transmitting member.
A force-measuring device is disclosed in U.S. Pat. No. 4,804,053, where the force is transmitted by means of so-called rocker pins or self-aligning struts. If they are designed with the appropriate dimensions, these self-aligning struts have the property that they position themselves on their own in the direction of the force flow path. For example in a weighing station for vehicles, this self-aligning property has the effect that the weigh bridge always settles into a position where it is free to swing laterally, although it will normally bump against the lateral stops when the vehicle to be weighed is driven onto the weigh bridge. As the force-transmitting members always align themselves in the direction of the force flow, there is no opportunity for transverse forces to occur and consequently, a parasitic leakage of the measurement force is avoided.
The aforementioned devices work well under normal conditions, but under difficult operating conditions they can no longer prevent, or will not sufficiently prevent, the occurrence of a force shunt, for example under loads that stress the device to its limits or in abnormal situations such as a faulty installation of the force-measuring device, incorrect operation, or an excessive accumulation of dirt. As a consequence, force shunts with their associated measurement errors can occur in spite of the aforementioned preventive measures.
The present invention is therefore directed to a method of monitoring the condition of a force-measuring device, specifically a balance, and to providing a suitable force-measuring device, whereby a simple and cost-effective design concept and operation of the measuring device becomes achievable, while stringent requirements in regard to measurement accuracy and stability are simultaneously met.
This task is solved by a method, a force-measuring device and a force-measuring module of the present invention.